The present invention relates to electro-erosion machining, or electrical discharge machining (EDM). Electro-erosion machining is influenced by any one or more of several parameters of operation, of a physical as well as chemical nature. The adjustment of the diverse machining parameters, such as the average machining current, the duration of the pulses and the rate of renewal of the dielectric fluid, is the result of a compromise between the rate of material removal from the workpiece, the electrode tool wear and the workpiece surface finish. It is known, for example, that the rate of wear of the electrode tool, especially when the electrode tool is made of copper, is influenced primarily by the duration and intensity of each electrical discharge, and secondarily by the duration of the cut-off time interval between two consecutive discharges and the physical and chemical states of the dielectric fluid. A decrease in the cut-off time interval causes a decrease in the rate of electrode tool wear, but correspondingly causes an increase in the contamination rate of the dielectric fluid and an increase of the number of abnormal discharges.
Means have been provided in the past to take advantage of those peculiarities in electrical discharge machining such as, more particularly, means consisting in controlling the duration of the time interval between two consecutive voltage pulses or in slowly varying the time interval as a function of the machining conditions (U.S. Pat. No. 3,614,368). It is also known to periodically apply high frequency relaxation pulses across the machining gap such as to obtain an improved deionization of the machining fluid (U.S. Pat. No. 3,056,065). Other processes attempt to avoid an excessive contamination of the machining fluid, for example by periodically interrupting the current pulses at a low frequency (1 to 2 Hz maximum) or in periodically withdrawing the electrode tool and increasing the fluid flow (Swiss Patent No. 414,034). It has also been proposed to apply across the machining gap series of pulses consisting of nominal current pulses of a variable amplitude greater than zero, separated by long duration pulses of greater amplitude (German Patent No. 1,165,242).
The processes have the common inconvenience of modifying the machining characteristics when the value of the average current is changed as a result of a switch-over to particular machining conditions. It is known that it is desirable to maintain constant the energy provided by each electrical discharge by applying across the machining gap current pulses of constant duration and amplitude. In order to decrease, for example, the average machining current, it is known to increase the time interval between two consecutive pulses, which results in changes in the machining characteristics and, more particularly, which increases the electrode tool wear. If, on the contrary, the machining conditions are such that the time interval between two consecutive pulses is decreased in order to increase the average machining current, such decrease of the time interval causes an increase of the machining fluid contamination, and an abnormal and destructive electrical discharge rate.
The present invention provides a novel process and apparatus permitting to modify the average value of the machining current in an EDM operation, without changing the machining characteristics such as to integrally maintain the advantages procured by constant energy electrical discharges. According to the present invention, the average machining current is controlled by providing current cut-off time intervals between consecutive pulse trains at an average frequency of at least 200 Hz and in varying the number of the pulses or the duration of the pulses within one pulse train and/or at least the longer one of the cut-off time intervals between two consecutive voltage pulses.
The invention achieves the result of decreasing the average machining current without increasing the electrode tool wear rate or, alternatively, of permitting to use a larger current than conventionally permissible in a given machining situation. The present invention improves sensibly the rate of material removal from the workpiece, expressed in amount of material removed per minute and per amp. The remarkable results achieved by the invention are attributed to the fact that the high frequency occurrence of the cut-off time intervals between two consecutive pulse trains influences favorably the random space distribution of the consecutive discharges and improves the random displacement of the discharge zone location. For example, the novel process of the invention may be used for cutting the average machining current in half by providing current pulses of 40 .mu.s, separated by current cut-off time intervals of 20 .mu.s between consecutive pulses, and by providing between each series, or train, of 12 of such pulses a cut-off time interval of the order of 1200 .mu.s. Another example of application of the process of the invention consists of interposing between 3 current pulses of a duration of 330 .mu.s each, separated by cut-off time intervals of 50 .mu.s, a cut-off time interval of 3300 .mu.s. In the latter case, the average current has been decreased by a ratio of 1 to 3. The process can be used for varying the average current in much greater proportions, such as, for example, in a ratio of 1 to 8.
It is also advantageous to use the process of the invention in combination with pulses having a high ionization voltage, such as 250 to 300 volts, or more. In that case, it is possible to decrease the nomial value of the cut-off time interval between two consecutive pulses to a value of a few .mu.s, which permits to machine with an increased average current without increasing the wear of the electrode tool.